1. Field of the Invention
The present invention relates to a method of fabricating an optical fiber preform by hydrolyzing a glass raw material in flame to generate glass particles and depositing the glass particles on a rotating starting rod to fabricate an optical fiber preform and a burner therefor.
2. Description of the Related Art
Up until now, various methods have been proposed for fabricating optical fiber preforms. Among these methods, an Outside Vapor Phase Deposition (OVD) Method, in which glass particles generated in burner flame are adhered and deposited on a rotating starting rod while relatively reciprocating the burner and the starting rod to synthesize a porous preform (i.e., soot preform) and the preform is dehydrated and sintered in an electric furnace, is widely used, because the method can make it relatively easy to fabricate an optical fiber preform having a desired refractive index profile and can mass-produce a large-diameter optical fiber preform.
For example, methods disclosed in Japanese Patent Application Laid-Open Nos. H04-243929 (1992) and H10-101343 (1998) and the like eject a glass raw material such as silicon tetrachloride, a burnable gas and a combustion assisting gas from a burner having a coaxially multiple tube structure, in which a plurality of nozzles are coaxially arranged, so that glass particles are generated in burner flame and deposited on a starting rod.
In order to improve the productivity of optical fiber preforms, it is necessary to enhance deposition rate of glass particles. However, the rise of a feed rate of the glass raw material to enhance the deposition rate of the glass particles produces decreases deposition efficiency of the glass particles on a starting rod so that the production cost of the optical fiber preforms increases. In addition, the decrease of the deposition efficiency increases an amount of floating glass particles in a reaction chamber which have not been deposited on the starting rod. As a result, the floating glass particles in a reaction chamber adhere to the surface of soot deposited on the starting rod, so that bubbles maybe formed in the optical fiber preform after it is transparently vitrified.
The reason of the decrease of the deposition efficiency is that the rise of the feed rate of the glass raw material gas makes flow velocity of glass particles greater so that the time required for the glass particles reaching the surface of the soot becomes too short.
A glass raw material gas is usually ejected from an ejection nozzle arranged burner having a coaxially multiple tube structure and glass particles are generated through a hydrolysis reaction of the glass raw material in flame. An enlargement of a diameter of the ejection nozzle for ejecting glass particles reduces a flow velocity of glass particles thereby time required for the glass particles reaching the surface of soot becomes longer. However, the thickness of the ejected glass raw material flame in a radial direction of the nozzle increases, so that the reaction at the center portion of the glass raw material flame is retarded and the resultant deposition efficiency is not improved.
Japanese Patent Application Laid-Open No. H04-243929 (1992) discloses a burner having a coaxially multiple tube structure of which an ejecting nozzle for ejecting a glass raw material is arranged between an ejecting nozzle for a combustion gas and an ejecting nozzle for an oxidation gas. In the burner, however, the combustion gas (H2) and the oxidation gas (O2) are ejected so as to sandwich the glass raw material gas therebetween so that a reaction of forming H2O slowly progresses. Accordingly, a generation of SiO2 slowly progresses.
Japanese Patent Application Laid-Open No. H10-101343 (1998) discloses a burner having a plurality of nozzles for ejecting a glass raw material, respectively. However, the burner has a complex structure so that it is difficult to mass-produce the preform with reasonable cost and a stable quality.